The present invention relates to a polishing apparatus for polishing a workpiece to be polished, such as a semiconductor wafer and the like, and more specifically, to a polishing apparatus having a function of detecting an escaping/slipping-off of a workpiece from a workpiece holding mechanism during polishing.
In recent years, as a level of micro-miniaturization and high-integration of semiconductor device has progressed, a space between wirings has become much narrower. Especially, in photolithography with a line width equal to at most 0.5 μm, a shallow focal depth thereof requires a high level of flatness in a plane on which an exposing apparatus forms an image. To realize such high level of flatness, a polishing apparatus has been broadly employed to provide an effective polishing operation.
This type of polishing apparatus typically comprises a polishing turntable with a polishing cloth affixed on a top surface thereof, and a top ring body. The turntable and the top ring body are driven to rotate at independently determined revolving speeds. A substrate to be polished, which is held in the top ring body, is pressed against the polishing surface of the turntable. A surface of the substrate to be polished is polished into a flat and mirror-finished surface while supplying an abrasive liquid or slurry to the polishing surface. After this polishing operation is finished, the substrate to be polished is removed from the top ring body and transferred to a subsequent process, a cleaning process for example.
The polishing apparatus, however, has suffered from a problem in that the substrate to be polished is occasionally cracked during polishing, and fragments of this broken substrate are dispersed across the polishing cloth. If such a polishing cloth having the fragments of the broken substrate dispersed thereacross is reused, the polishing cloth could cause scratches on a surface of a substrate to be polished. Due to this, the polishing cloth has to be replaced with another each time a substrate to be polished is cracked.
There has been still another problem, even in a case of no cracking of a substrate to be polished, in that the substrate to be polished occasionally slips out of the top ring body. In this case, if the substrate to be polished is composed of fragile material as represented by a silicon wafer and the like, the substrate could impinge upon a wall surface of a casing covering a turntable and occasionally be damaged, such as by being chipped, in a peripheral region of the substrate to be polished. If this damaged substrate is to be polished again, only a light load applied to the vicinity of the damaged area could crack the substrate.
To address the above-mentioned problems, a polishing apparatus as disclosed in Japanese Patent Laid-open Publication No. 2001-96455 comprises a sensor of an ultrasonic type, which is disposed on an outside of a top ring to measure a distance to a top surface of a turntable. When the distance to the top surface of the turntable, which is measured by the sensor of the ultrasonic type, is changed due to intervening of a substrate to be polished that happens to slip out onto the surface of the turntable to be measured, the sensor detects this as an abnormal polishing or a slipping-off of the substrate to be polished.
Further, another polishing apparatus as disclosed in Japanese Patent Laid-open Publication No. 2001-96455 includes a condenser comprising electrode plates disposed to sandwich a substrate to be polished, which is held by a top ring, from both sides thereof. Alternatively, the polishing apparatus may include a condenser comprising electrode plates disposed in locations to sandwich a substrate to be polished which happens to slip out of the top ring. A constant voltage is applied to such a condenser, so that abnormal polishing or slipping-off of the substrate to be polished may be detected based on a current flowing through the condenser.
Still further, another polishing apparatus as disclosed in Japanese Patent Laid-open Publication No. 2001-96455 includes a contact element to be in contact with a turntable at a location defined on an under surface or a periphery of a top ring. A current is applied between this contact element and a turntable surface, so that abnormal polishing or slipping-off of the substrate to be polished may be detected based on a change in a current flowing therethrough.
However, either one of the above-described detection methods for determining abnormal polishing or slipping-off of a substrate to be polished has also suffered from a problem in that it is susceptible to effect of noise and the like, and thus is not reliable. In addition, any one of these methods has been associated with another problem in that it takes a long time to execute a signal processing and to finally determine whether or not this processed signal indicates abnormal polishing or slipping-off of a substrate to be polished, thereby disadvantageously leading to a state as allowing the substrate to be polished to impinge against, for example, a wall surface of a casing surrounding a turntable during waiting for an appropriate action, such as stopping of a top ring and/or the turntable, to be taken.